have not tried, but an idea: NLogManager has a method GetLogger(string name). As I understand, you can have several Logger instances with different names resulting in different files.
Then you can try the following. define a class field of type Logger in the class StoreSCP (the class deriving from DicomService). In the method
OnReceiveAssociationRequest(DicomAssociation association)
you could then initialize this field with _logger = nLogManager.GetLogger(association.CallingAE). by that you have a Logger instance for every calling AETitle.
Does this idea help/work?